This invention relates generally to gas generation and, more particularly, to devices and methods for inflating inflatable devices such as inflatable vehicle occupant restraints of respective inflatable restraint systems.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an “airbag cushion,” that is inflated or expanded with gas such as when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the system, the cushion begins to be inflated, in a matter of no more than a few milliseconds, with gas produced or supplied by a device commonly referred to as a “gas generator” or an “inflator.”
Many types of inflator devices have been disclosed in the art for the inflating of one or more inflatable restraint system airbag cushions. Prior art inflator devices include compressed stored gas inflators, pyrotechnic inflators and hybrid inflators. Unfortunately, each of these types of inflator devices has been subject to certain disadvantages such as one or more of having a greater than desired weight, requiring more than desired space or volume, and producing undesired or nonpreferred combustion products in greater than desired amounts, for example.
Vehicular inflatable restraint systems and their manufacturers typically face the objectives of increased airbag performance and safety while also seeking to reduce or minimize costs. To this end, significant research efforts and expenditures have gone into increasing the functionality of the propellant or gas generant material for use in airbag inflators and reducing the cost with an end goal of simultaneously improving performance and reducing the cost for the entire inflatable restraint system.
Improved performance for pyrotechnic-containing gas generators or inflators may be achieved in a variety of ways, many of which ultimately depend on the gas generant formulation or composition to provide desired properties. Ideally, a gas generant provides or results in sufficient mass flow of gas in a desired time interval to achieve the required work impulse for the associated inflating device. The temperature of the produced gas influences the amount of work the generant gases can do. Traditionally, the production of high temperature gases can be troublesome such as to typically require that managing of the thermal energy during the inflation event.
In general, the burn rate for a gas generant composition can be represented by the equation (1), below:Rb=k(P)n  (1)
where,                Rb=burn rate (linear)        k=constant        P=pressure        n=pressure exponent, where the pressure exponent is the slope of a linear regression line drawn through a log-log plot of burn rate versus pressure.In such a burn rate equation, the pressure exponent, “n” is an indication of the pressure sensitivity of the burning rate. That is, a composition that exhibits a larger pressure exponent indicates that the burn rate of the composition is more highly sensitive to the surrounding pressure.        
Gas generant materials or compositions that produce high temperature gases, such as evidenced by compositions having a high flame temperature, with high gas yields can find advantageous application in inflators that employ another working fluid such as a stored gas or liquid, such as in various hybrid inflators, for example. More particularly, in such inflator applications only a relatively small amount of a high temperature, high gas yield gas generant material or composition is required to heat the working fluid to a desired and useful temperature resulting in overall cost and weight savings for the inflator and the inflatable restraint system as a whole as compared to inflators and systems employing conventional cooler burning gas generant compositions.
Thus, there is a need and a demand for gas generant compositions, particularly, gas generating pyrotechnic compositions that produce high yields of gas at high temperatures with rapid burning rates to maximize the volumetric performance of the composition.